Electronic systems often include various heat generating devices such as microprocessors, hard disc drives, graphics processors, memory chips, and power supplies, and power converters, just to name a few. Waste heat generated by those devices must be effectively removed by a thermal management system or those devices will fail or otherwise become unreliable due to operating temperatures that exceed design limits. Typically, waste heat is removed from an enclosure that houses the devices through several paths. In many cases the system includes a fan that moves air over the devices to remove waste heat through convection cooling. A significant fraction of the waste heat can be removed using convection cooling. Another significant heat removal path is through the materials that comprise an enclosure that houses the heat generating devices. Waste heat in an interior of the enclosure is thermally conducted to an outer surface of the enclosure and into the ambient air or some surface in contact with the enclosure. As a result, a temperature of the outer surface is rarely uniform and hot spots often exist adjacent to a heat source.
One disadvantage to using the enclosure as a heat removal path is that some areas of the enclosure can be quite warm to the touch and in some cases a surface temperature of the exterior of the enclosure can reach uncomfortable levels. Enclosure temperatures are particularly important in portable electronic devices such as laptop computers and tablet PC's where a user holds the device or rests the device on their lap or on an arm. The human body provides an efficient liquid cooled environment that tends to draw heat out of electronic enclosures. Although it is desirable to remove heat from a system housed in an enclosure by using the enclosure itself as a conductive path, it is also desirable to keep surface temperatures of the enclosure within comfortable levels for the user.
As an example of a prior heat removal approach 200, in FIG. 1a a heat source 205 that dissipates a substantial amount of heat h is positioned in contact with or in close proximity an interior surface 201i of a laptop computer enclosure 201. The heat source 205 can be a graphics chip, a North Bridge chip, or a microprocessor, for example; however, for the purposes of experimental verification, if FIG. 1a the heat source 205 is a series of surface mount resistors connected with a power source 207 for supplying a current i that can be varied w to adjust the amount of heat h generated by the source 205 (i.e. the power in Watts). A thermocouple 209 connected with the heat source 205 can be used to measure a temperature of the heat source 205 using a thermometer 211.
Typically, plastics including a Polycarbonate (PC) or a PC/ABS material are used for the enclosure 201 because plastic is a good electrical insulator, has a low weight compared to metals, and can be manufactured at a low cost. The enclosure 201 can also be made from a metal such as aluminum (Al), for example. Optionally, an air gap (not shown) between the interior surface 201i and the heat source 205 allows an airflow from a fan to cool the heat source 205 and to remove some of the heat h from the interior of the enclosure 201. However, some of the heat carried away by the air flow is thermally transferred to the interior surface 201i. Additionally, radiant heat from the heat source 205 is also transferred to the interior surface 201i. Consequently, the heat h is thermally transferred from the interior surface 201i to an exterior surface 201e of the laptop computer resulting in a high surface temperature (e.g. a hot spot) at the exterior surface 201e. A user of the laptop computer may feel discomfort when the exterior surface comes into contact with their body.
The above mentioned discomfort is due to the heat h being concentrated in a portion of the enclosure A that is adjacent to the heat source 205. Essentially, the heat h that is thermally transferred from the interior surface 201i to the exterior surface 201e does not evenly spread out over the entire exterior surface 201e. Instead, the heat is mostly concentrated in a hot spot A over the heat source 205 and that hot spot A is the cause of the discomfort to the user. One need only rest a prior laptop computer on a surface of a table, and after short period of time, move the laptop aside and feel that only a portion of the surface is warm to the touch because the heat from the hot spot A was concentrated over the warm portion of the surface of the table.
In many applications a thin layer of an electrically conductive material 203, such as a metal, for example will be bonded with the interior surface 201i to serve as an EMI shield. As an example, a steel stamping is often bonded with the plastic enclosure 201. In either case, the plastic and the steel have an equally bad thermal conductivity along all three axes (x-y axis, z-axis). Accordingly, at a point B on the exterior surface 201e, a temperature on the exterior surface 201e is substantially cooler than at the hot spot A because a majority of the heat h is thermally conducted into the z plane instead of the x-y plane.
As an example of a hot spot generated by the prior configuration of FIG. 1a, in FIG.1b, with the heat source 205 generating about 2.0 W at a temperature of about 122.9° C. above an ambient temperature of about 25° C. Infrared thermal imagery of the exterior surface 201e depicts a temperature of about 97° C. at a hot spot A (see 251A) and a temperature of about 22.6° C. at a point B (see 251B), with a difference of 74.4° C. between the A and B temperatures. Most of the heat h is concentrated over the hot spot A as depicted by the heavy solid arrows, and to a lesser extent, some of the heat h spreads a short distance from the hot spot A as depicted by the dashed arrows. Consequently, the hot spot A can be a source of discomfort when the exterior surface 201e comes into contact with a users body.
As another example of a prior approach for reducing hot spots, in FIG. 1c a layer 221 of a thermally insulating material is bonded with the interior surface 201i and the heat source 205 is in contact with the insulating layer 221. The material for the layer 221 is a PTFE bound silica aerogel insulation material with a thickness of about 0.75 mm and including a very low thermal conductivity along all three axes (x-y, and z).
In FIG. 1d, at 2.0 W power dissipation from the heat source 205, even with an insulator 221 between the heat source 205 and the exterior surface 201e, the thermal image indicates a temperature of about 82.1° C. at a hot spot A (see 252A) and a temperature of about 25.3° C. at a point B (see 252B), with a difference of 56.8° C. between points A and B. Although, the temperature at the hot spot A is lower and the temperature differential is lower when the insulating layer 221 is used, the temperature of the heater 205 actually rises to about 134.4° C. above ambient temperature, an increase of 11.5° C. when compared to the 122.9° C. in the configuration of FIG. 1a where no insulation is used. Consequently, with a thermal insulating material, an actual operating temperature of the device the heater 205 simulates (e.g. a graphics chip or a North bridge chip) would get hotter which is not good for device reliability.
In FIG. 2a, a prior heat removal approach 200 can also include the addition of a heat spreader material 213 positioned between the enclosure 201 and the heat source 205. There may be an air gap (not shown) between the heat spreader 213 and the heat source 205. Although heat spreader materials can have high thermal conductivities in all three planes (x-y and z), the material selected for the heat spreader 213 has its highest thermal conductivity in the x-y plane and a more modest thermal conductivity in the z plane. The heat spreader 213 serves to thermally conduct heat h away from the heat source 205 and distribute a portion of the heat h along the enclosure 201 so that the temperature at the hot spot A is reduced. In FIG. 2b, with the heat source 205 dissipating 2.0 W, thermal imagery of the enclosure 201 with the heat spreader 213 depicts the hot spot A at about 41.9° C. (see 253A) and the point B at about 32.3° C. (see 253B), a difference of about 9.6° C. The heat source 205 was at a temperature of about 52.7° C. above ambient temperature. Although this is an improvement over the thermal images of FIGS. 1b and 1d, the hot spot A in FIG. 2b is still warmer than desired and may result in discomfort to a user.
Moreover, with the heater 205 set to a higher power dissipation of 5.0 W in FIG. 2c, the thermal image depicts the hot spot A at about 61.8° C. (see 255A) and the point B at about 44.5° C. (see 255B), a difference of about 23.6° C. The heat source 205 was at a temperature of about 131.4° C. above ambient temperature. Accordingly, the heat spreader 213 alone cannot reduce the temperature of the hot spot A to comfortable user levels. Furthermore, at the higher power levels (i.e. 5.0 W) that can be expected from components such as graphics chips and North bridge chips, the temperature of the heat source 205 above ambient temperature more than doubles and the hot spot A temperature increases by 26.2° C.
Consequently, there exists a need for a device for uniformly spreading heat transferred to a surface of an enclosure over a large region so that hot spot regions are reduced and user discomfort is reduced because the heat is uniformly distributed over the surface. There is also a need for a device for equalizing temperatures on a surface of an enclosure that does not increase a temperature of a heat source. Finally, there is a need for a device for equalizing temperatures on a surface of an enclosure that can reduce a temperature of a heat source while evenly distributing heat along a surface of the enclosure thereby reducing hot spots.